It is necessary to wear a crash helmet when riding a bicycle or a motorcycle. A ring band is provided in the crash helmet to be located and matched at the brow of a user. If the ring band is not an adjustable structure, it cannot apply to people having different sizes of heads. An adjustable ring band has been proposed, which has a connection seat and a cover body connected to two ends of the ring band. Although it has a hidden adjustment structure, because the connection seat needs to be fixedly joined with the cover body, a joining structure between them must be provided. Because the cover body is used to cover the connection seat, the main body of the connection seat and the main body of the cover body must have the same size. Therefore, some material is wasted. Moreover, the two ends of the ring band are locked at connection posts of an adjustment seat of the connection seat via through holes. The fixing effect of this kind of joining is not good so that other processing procedures must be matched, e.g., riveting the connection posts at the ring band. Therefore, the production cost is increased, and the manufacturing and assembling time is also increased. Additionally, the locating of a prior art adjuster can only be controlled by sense, resulting in inconvenient use. The present invention aims to resolve the above problems in the prior art.